The demand for removing salt from brackish and saline waters continues to increase, as water demand outstrips net rainfall for half the world's population, causing more reliance on groundwater withdrawal, seawater, agricultural water recovery, and potable water reuse. The fastest-growing technology used to meet these demands is reverse osmosis (RO), which is the most efficient technology for most water sources. Increasing RO energy efficiency is vital to reducing the operating cost and carbon intensity of desalination. While many research areas, such as membrane development, have reached diminishing returns in RO efficiency, there are still significant gains to be made in process design. Herein, we examine the efficiency improvements made possible by switching to time-variant RO systems and other systems for liquid mixture/solution separation.
Reverse-Osmosis (RO) Desalination:
While RO is the most energy-efficient desalination process under most conditions, further improvements on efficiency are advantageous for minimizing the CO2 impact of the energy requirements for RO, allowing for RO where power production is limited, reducing energy costs, and improving public acceptance of desalination.
Batch reverse osmosis technologies are configurations that vary their salinity over time by recycling brine. Batch technologies have also shown impressively robust resistance to membrane fouling, although an explanation for this is lacking from the literature. One of the most rapidly growing technologies is a semi-batch RO process, called CCRO, or closed circuit reverse osmosis (and trademarked as CCD, or closed circuit desalination).
Closed Circuit Reverse Osmosis (CCRO):
Closed circuit reverse osmosis is a semi-batch process in which feed is continuously added to the system over time. In a CCRO system, feed water is pumped through the membrane module, where pure water passes through the membrane while the remaining solution is concentrated. The brine is then mixed with fresh, high pressure feed water and returned to the membrane module to be further concentrated. To account for this increasing concentration, the pressure of the system is increased over time. Once the desired amount of permeate has been produced, a valve is opened, and the system is refilled with feed in preparation for a new cycle. Several designs have been proposed historically in the patent literature for CCRO (U.S. Pat. Nos. 4,243,523, 4,814,086, and 4,983,301).
CCRO has potential advantages in terms of both fouling resistance and energy consumption. CCRO has been shown to be fouling resistant and has been tested to recoveries as high as 97%, although 88-92% is more typical. CCRO needs less energy than continuous RO because CCRO varies the pressure over time, which lets it stay closer to the osmotic pressure of the feed. In comparison, continuous RO sets the pressure everywhere above the maximum osmotic pressure of the outlet brine. However, one pitfall of CCRO is that it continuously mixes brine with incoming feed, which generates entropy and limits the efficiency of the process.
Past models of CCRO have modeled the process as a series of steady cycles with step pressure increases in between. This is a tolerable approximation for high recoveries (large numbers of cycles) with the cycles generally capturing the performance variation in time. However, these models do not capture the salinity profiles within the module. Furthermore, the discrete nature of the cycles prevents these from models from being used to study batch RO systems, which reach high recovery in few cycles. In order to improve accuracy and make a fair comparison to the batch process, we model CCRO as a temporally- and spatially-varying process that is modeled by numerically solving finely discretized equations, rather than a simple analytical model of a few cycles.
Other Batch Configurations:
With respect to RO, the term “batch” has been used to indicate several configurations. Herein, “batch RO” signifies that RO brine is recirculated through the RO membrane module without incorporating any fresh feed. On the other hand, the term “closed circuit RO” is used to refer to configurations where RO brine is mixed with feed and re-circulated in a continuous manner, which is termed a “semi-batch” process due to the continuous feed addition. While the idea of a completely batch RO was proposed in U.S. Pat. No. 4,983,301, the concept was further developed more recently by various inventors. Oklejas proposed systems where the brine recirculation was integrated within the RO pressure vessel (U.S. Pat. No. 8,808,538). Batch RO systems have been reported to have problems maintaining permeate quality [R. L. Stover, “Industrial and brackish water treatment with closed circuit reverse osmosis,” 51 Desalination and Water Treatment 1124-1130 (2013)], so systems with variable feed pressure have also been proposed (see U.S. Pat. No. 7,892,429).
While others have explored batch RO processes, published studies on the modeling and performance of batch RO systems are limited. Barello conducted experiments on a batch RO process to study the influence of pressure and feed salinity on the water permeability constant of the membrane [M. Barello, D. Manca, R. Patel, and I. Mujtaba, “Operation and modeling of ro desalination process in batch mode,” Computers & Chemical Engineering, 2015]. Tarquin and Delgado reported that batch RO may be specially resistant to fouling and scaling based on experiments in which fouling was not observed even with brackish water under high concentrations of silica and calcium sulfates at 90% recovery [A. Tarquin and G. Delgado, “Concentrate enhanced recovery reverse osmosis: a new process for RO concentrate and brackish water treatment,” Proc. American Institute of Chemical Engineers Meet., Pittsburg, Pa., USA, October, American Institute of Chemical Engineers, Paper 272277 (2012)].
Membrane fouling can lead to declining flux, increasing stream-wise pressure drop, and changes in salt permeation. These changes, in turn, affect water cost through pretreatment requirements, increased energy consumption, frequent membrane cleanings, and eventually membrane replacement. Resistance to fouling of various types, including inorganic, organic, and biological, is thus a common theme in desalination research.
Inorganic fouling, or scaling, is of particular importance in low salinity water desalination. Khan, et al., in “How different is the composition of the fouling layer of wastewater reuse and seawater desalination RO membranes?,” 59 Water Research 271-282 (2014), harvested foulant layers from RO membranes used to treat seawater and secondary wastewater effluent in a pilot plant, and found that, although organic foulants dominated in seawater RO and on the first membrane of wastewater RO, inorganic foulants comprised 88.9% by mass of the foulant layer on the last membrane in the wastewater RO train. The high degree of brine concentration due to the high recovery typical of low-salinity water desalination tends to concentrate inorganic foulants, such as calcium carbonate, to beyond their saturation limits, causing scale on the later membranes.
The susceptibility of RO membranes to damage by fouling has prompted the development of other processes, such as membrane distillation and forward osmosis, which are thought to exhibit greater resistance to fouling. However, the higher efficiency of RO makes it worthwhile to consider modifications to the RO process that could lead to improvements in fouling resistance. Stover has proposed that CCRO can reduce fouling and scaling through the time-variation of water composition at the membrane [R. Stover, “Evaluation of closed circuit reverse osmosis for water reuse,” in Proc. 27th Annual Water Reuse Symp., Hollywood, Fla., USA, September, Water Reuse Association, Paper B4-2(2012)]. Herein, we examine the cycle time of CCRO as well as batch operation to identify potential gains in scaling resistance through these time-variant RO processes.